A great deal of power and heat productions is based on the combustion of both fossil and non-fossil carbonaceous fuels such as coal and oil, and peat and biomass, respectively. The production of power or heat may also be based on the combustion of the gas obtained at gasification of the above-mentioned raw materials. Although gasification constitutes an extra step in the power or heat production chain, it entails considerable advantages that justify this "detour", particularly from the environmental and efficiency aspects. Great efforts are being made to optimize the gasification process to make it still more efficient and therefore more competitive. The conditions for optimization of the gasification process are quite different from those applicable for combustion processes. This is accentuated if the gasification process also includes recovery of inorganic chemicals from the raw material which is difficult to optimize.
In general the gasification of a carbonaceous raw material, i.e. the thermal disintegration when oxygen is supplied under-stoichiometrically, comprises exothermic oxidation reactions and endothermic gasification reactions. Heat is emitted from the exothermic oxidation reactions, which is necessary to produce the endothermic gasification reactions. In principle it can be said that at a stoichiometry of 0.50 half the raw material is combusted exothermically while the other half is gasified endothermically.
In many processes it is desirable to minimize the stoichiometry for other reasons also, i.e. have as low a supply of oxygen as possible, in order to obtain a sufficiently reducing environment, for instance. However, a general problem is then that the energy (heat) produced through the partial combustion is not sufficient to effect the desired degree of gasification, i.e. usually total gasification of uncombusted organic raw material present. This problem can be alleviated by supplying external energy so that a desired proportion of gasification is obtained. There are several methods of supplying external energy without supplying extra oxygen, such as by pre-heating oxygen-containing air supplied or by means of an energy-rich gas heated in a plasma generator. One such method is known through SE-B-448 007 in which external energy is supplied to a gasification reactor by means of a plasma generator. The need of the exothermic oxidation reaction of the black liquor decreases with increasing plasma effect, thus enabling lower stoichiometry, which results in an increased energy content in the gas produced. However, the use of a plasma generator is complicated and expensive. Furthermore, the plasma generator utilizes high quality, expensive electric energy. To a certain extent the same drawbacks exist when using equipment for pre-heating air.